The goal of this investigation is to more clearly defme M- 1 and M-2 macrophages and how they influence immune responses. The concept of M-1 and M-2 fomented from observations in this laboratory that with the same stimuli, macrophages from certain mice (C57BL/6, B1OD2) preferentially metabolize argimne to NO while other mice (Balb/c, DBAI2) increase ornithine production. NO inhibits cell replication while omithine (via polyamines) promotes cell replication. Therefore, M-1/M-2 does not simply represent activated or unactivated macrophages, but macrophages that have upregulated qualitatively different metabolic programs. M-1/M-2 propensities are also observed in C57BL/6 and Balb/c SCID mice and are thus independent of T or B lymphocytes. Indeed, other evidence indicates that M-1/M-2 macrophages can shepherd T lymphocytes into Thi or Th2 responses, respectively. Macrophages are a major precursor of dendritic cells, which have been reported to influence the Thl/Th2 balance. Therefore, our results suggest that macrophages play much a more important role in orchestrating immune responses than is currently appreciated. The Main Hypothesis is that M-1 is associated with destructive products and Thl responses, while M-2 is associated with the constructive products involved in healing/regeneration and with Th2 responses. It is also hypothesized that overexpression of the M-l phenotype (e.g. NO) can inhibit specific immune responses. To test these hypotheses Specific Aim 1 will more clearly define products and properties of M-1/M-2 macrophages including a) arginine metabolites (NO/ornithine); b) oxygen radicals (O2-, ONOO-, H2O2); and c) cytokines/growth factors (e.g. IL- 12, IFN-g, IL-8, IFN a/b, macrophage stimulating protein). Specific Aim I will also determine if M-1/M-2 macrophage phenotypes are expressed at the single cell level using double color ELISPOT. Specific Aim 2 will determine how M-l/M-2 macrophage responses influence non-specific or specific immune responses in vivo. Specific Aim 2a will determine how M-1/M-2 macrophage responses influence non-specific inflammation associated with injury/danger. Specific Aim 2b will compare innate immunity in M-l/M-2-dominant mice. Specific Aim 2c will determine how M- 1 /M-2 macrophages influence tumor specific or allo specific immune responses. M1 (C578L/6) and M-2 (Balb/c) SCID mice will be used extensively as test subjects and as sources of macrophages for adoptive transfer to directly determine effects due to M-1/M-2 macrophages. Together, this investigation will result in a major increase in our understanding of M-1/M-2 macrophages and how they influence immune responses.